My invention relates to axial flow reversible fans and, in particular, to those fans adapted for use as plug units in high temperature environments such as heat treating furnaces.
It is known in the art that the use of inlet and outlet vanes with axial flow fans will improve the operating characteristics of the fan. Inlet vanes are employed to change the direction of the incoming fluid so that the velocity vector of the fluid relative to the impeller blade is always tangent to the blade. This will result in fluid flow that is parallel to the leading edge of the impeller blade and tends to decrease flow separation, also known as "shock losses." Similarly, outlet vanes are used to deflect the fluid as it exits the impeller blades and remove the tangential component of the exiting fluid velocity vector so that the helical swirl of the fluid is reduced and straight-line flow is promoted. Like the impeller blades, the outlet vanes must be curved so that the velocity of the exiting fluid relative to the vanes is tangent to the vane to minimize shock losses around the vane.
However, these systems usually employ impeller blades having a concavo-convex type cross section, as in British Pat. No. 515,469, which tends to improve fan performance for fluid flow in one direction but seriously impairs fan performance when the fan is reversed and the fluid flows in the opposite direction.
It is also known in the art to construct a reversible axial fan having a set of concavo-convex vanes on either side of the impeller. For example, the Agushev et al. U.S. Pat. No. 3,820,916 discloses a fan comprising two impellers and two sets of guide vanes arranged along the drive shaft of the fan so that the sets of vanes are separated by an impeller. Fans of this type generally are designed to operate under normal temperatures and pressures in a noncorrosive environment and have vanes and impeller blades whose pitch is adjustable to meet the required performance characteristics for forward and reverse flow. However, these fans have several significant disadvantages. In order that the fan vanes might be adjustable without dismantling the fan to adjust the pitch of the vanes, it is necessary to include in the fan construction cumbersome mechanical linkages which require periodic removal and disassembly to repair or maintain. In addition, the mechanical linkages require much space directly in the path of air flow through the fan and result in a significantly reduced air flow area for a given diameter fan. Since such reversible fans are designed to operate under a variety of required air flows and air velocities, the inlet and outlet vanes cannot compensate properly for the increase in impeller blade angular velocity with the radius of the impeller blade. As a result, inlet vanes can impart only an approximation of the proper prespin to eliminate shock losses and outlet vanes can only approximate the required pitch needed to decrease sufficiently the tangential component of the exiting fluid velocity vector and meet the exting fluid so that the velocity of the fluid relative to the vane is tangent to the vane.